1. Field of the Invention
The present invention relates to a method of producing a dither matrix for converting continuous tone images into pseudo-halftone images.
2. Description of Related Art
Image data representative of continuous tone images is generally constructed from about eight bit data indicative of 256 tone levels. However, output devices, such as printers, for producing visible images based on the image data are generally bilevel output devices. The bilevel output devices produce images by printing dots or non-dots on recording sheets. For example, the bilevel output devices produce binary images through selectively providing ink dots onto the recording sheets. The bilevel output devices are simple in construction and easy to control.
In order to record eight bit continuous tone images with the bilevel output devices, the eight bit data for each picture element (pixel) is compared with a single predetermined threshold value, thereby determining whether or not to provide ink onto the recording sheets. According to this method, however, it becomes impossible to reproduce the great variety of tone levels which are originally represented by eight bit data. This is because eight bit data can represent 256 tones per color, and therefore can represent 16,777,216 tones for three colors in total.
In order to solve this problem, there has been proposed a half-toning method for representing various tone levels through providing a corresponding number of dots within a certain amount of area.
This halftoning methods employ various types of operations such as an error diffusion operation, a random number threshold-using dithering operation, and an ordered-dithering operation.
Those conventional half toning operations suffer from the following problems.
When a continuous tone image is halftoned with using the error diffusion operation, a density value of each pixel of the continuous tone image is modified according to errors generated at neighboring pixels. The modified density value is compared with a predetermined threshold value. Based on the compared result, the pixel density value is converted into a binary value. The error or difference between the binary value-representing value and the modified pixel density value will be diffused to neighboring pixels. It, however, requires a long period of time to perform those calculation processings onto all the pixels of the image. The obtained binary images will suffer from undesirable textures.
The random number threshold-using dithering operation employs a dither matrix whose threshold value elements are constructed from random numbers. Generally, each continuous tone image, to be dithered with the dither matrix, is wider than the dither matrix. The dither matrix is repeatedly laid down over the continuous tone image in a periodic manner. A density value of each pixel of the continuous tone image is simply compared with a threshold value on a corresponding location of the dither matrix. When the pixel density is higher than the corresponding threshold value, the pixel is turned ON. When the pixel density is equal to or lower than the corresponding threshold value, the pixel is turned OFF. Accordingly, it is possible to perform the processing within a short period of time. However, because the threshold values of the dither matrix are determined by random numbers, the resultant binary image becomes noisy.
The ordered-dithering operation also employs a dither matrix whose threshold values are determined one by one spirally around the matrix center. The dither matrix is repeatedly laid down over the continuous tone image in a periodic manner. A density value of each pixel of the continuous tone image is simply compared with a threshold value on a corresponding location of the dither matrix. Dots in the resultant binary image, however, tend to be gathered around certain dots and are erroneously realized as large dots. The resolution of the resultant binary image becomes deteriorated. Bayer method, another method of producing an ordered-dither matrix, arranges the threshold values of the dither matrix according to another rule. The obtained dither matrix can more uniformly distribute dots on the resultant binary image. However, because the Bayer's dither matrix is produced still according to a fixed rule, undesirable textures are still generated in the resultant binary images. Especially, densities in the dark portions are insufficiently reproduced.
Because the dither matrix DM is replicated on the continuous tone image as shown in FIG. 1, a plurality of portions in the continuous tone image are successively converted into binary image portions. Accordingly, the resultant binary image will suffer from non-uniform colors or tones at edges BD1 and BD2 between the dither matrix-replicated portions. Accordingly, undesirable boundary lines such as white lines will tend to periodically appear in the resultant binary image.